Nowadays you can buy almost anything online and most of the online stores give commission to web sites which give them customers. The idea behind Shop to Donate is for a web site to give this commission to charity if you do your shopping using their links. Some Shop to Donate web sites have affiliate agreements with loads of different online shops and so they can also be called Charity Malls.
Let's say you want to buy a book from Amazon.com. You can buy the book directly from Amazon.com or you can buy it through Charity Cafe for example. The price is exactly the same and the product is exactly the same, but if you buy it using the links Charity Cafe, then Charity Cafe will receive a commission and donate that to charity. With Amazon.com for example the commission can be as high as 15% of your order value.
To get an idea of what kinds of things you can buy from a Charity Mall, visit the shopping page at Care2.com:
http://www.care2.com/shopping/shopping_categories_nogtu.html
Another good Charity Mall is called Igive.com:
http://www.igive.com
Here is a UK specific Charity Mall:
http://www.rectifi.org.uk
And here is a direct link to a charity mall from The Environment Site: